Introduction: The gut microbiota influences the severity of brain injury through regulation of intestinal immune homeostasis. However, molecular cues involved in microbiota-immune interaction after stroke are unknown. Metabolites have a pivotal role in connecting commensals to the immune response. Specifically, bacteria-derived metabolites of tryptophan catabolism activate the transcription factor aryl hydrocarbon receptor (AhR) which regulates T cell polarization directly or via tolerogenic function of dendritic cells (DCs).
Hypothesis: Since adaptive immune cells are involved in stroke, we hypothesize that modulation of tryptophan metabolism–orchestrated by bacteria–modifies T cell differentiation and influences stroke outcome.
Methods: Stroke is induced by the occlusion of the middle cerebral artery (MCAO) in C57BL6 mice. Blood and the distal small intestine are sampled for the detection of tryptophan metabolites and quantified by metagenomics or mRNA expression. Immune cells are isolated from the intestine and analyzed by flow cytometry. Stroke outcome is measured by infarct volumetry and behaviour tests.
Results: Tryptophan catabolism was augmented in mice subjected to stroke-induced dysbiosis–shown by a) an increase of the most proximal metabolite kynurenine and a concomitant decrease of tryptophan and b) an upregulation of mRNA expression of the rate-limiting enzyme involved in tryptophan’s breakdown. This was paralleled by an increase of AhR mRNA in the ileum after MCAO. To test the implication of AhR on the tolerogenic function of DCs, we used AhR-DCs knock-out mice. We found that CD103+CD11b+ DCs lacking AhR were downregulated after stroke compared to wild-type (WT) mice. Interestingly, polarization state of T cells was modified in DCs lacking AhR (regulatory versus Th17 cell ratio). This could influence the central nervous system immune response and induce protection after stroke, as suggested by a reduction of infarct volume observed 3 days after stroke in AhR-DCs knock-out mice compared to WT mice.
Conclusion: These findings suggest that stroke-induced dysbiosis leads to an increase of tryptophan catabolism, and that AhR depletion in DCs influences immune cell polarization and stroke outcome.